The invention relates to the general field of die bonding, and in particular to a flexible polymer lead-frame.
Traditionally, most integrated circuits (IC's) have been connected using three metal connections. First, the die is wire-bonded to a lead-frame. Wire bonding connects one end of a fine metallic wire to an IC contact pad. The other end of the wire is connected to a metal lead-frame. The IC and part of the lead-frame are then sealed into a molded package to produce the well-known IC package. The leads of the package are then soldered to the circuit to produce a third connection.
So-called Chip-On-Board methods are similar to wire bonding but without a lead-frame. A bare die is first attached to a circuit board with pads facing away from the board. A drawn wire is then metal bonded to each IC pad and to a corresponding circuit board pad, then the assembly is encapsulated.
Tape Automated Bonding (TAB) is another connection methodology which produces a similar result to wire bonding. Metal foil laminated on a substrate is typically etched in a patterned array to form traces on a dielectric film. The copper traces typically extend over an opening which is sized to be slightly larger than the die. These extended portions are sometimes called fingers. The die is placed in the opening so that fingers can be metal-bonded to die contact pads. Ends of the traces away from the fingers can be connected to a substrate using other methods. The bonds from the copper trace to the die are sometimes called inner bonds, while the bonds from the copper trace to the substrate are sometimes called outer bonds. TAB is similar to wire bonding in that one end of a metal wire (etched trace) connects to the die while another end is attached to a circuit board.
Yet another approach solders dies directly to ceramic circuit boards, and is sometimes called a "flip chip" process. Rounded solder bumps are specially fabricated on the chip contact pads. A rigid substrate is provided with a matched set of contact points. After placing the chip, bumps down, on the substrate, the solder is reflowed.
Anisotropic conductive adhesives (sometimes called Z-axis adhesives) displaying unidirectional conductivity have been reported for outer bonds in a TAB process. The adhesive consists of a dielectric thermoplastic adhesive and nickel conductive particles. See "Direct Chip Interconnect Using Polymer Bonding," K. Gilleo, 1989, presented at the IEEE 39th Electronic Components Conference.
Attempts have been made to use conductive adhesives to bond dies directly to substrates, although they are reportedly not commercially applicable. See "Direct Chip Interconnect Using Polymer Bonding," K. Gilleo, 1989, pp. 41-42. Significant difficulties are associated with applying viscous adhesive to very small bonding sites, especially when bonding site pitch (center to center distance) is less than about 250 microns.